Generally, well services include a wide variety of operations that may be performed in oil, gas, geothermal, or water wells, such as drilling, cementing, completion, and intervention. Well services are designed to facilitate or enhance the production of desirable fluids such as oil or gas from or through a subterranean formation. A well service usually involves introducing a fluid into a well.
Drilling is the process of drilling the wellbore. After a portion of the wellbore is drilled, sections of steel pipe, referred to as casing, which are slightly smaller in diameter than the borehole, are placed in at least the uppermost portions of the wellbore. The casing provides structural integrity to the newly drilled borehole.
The well is created by drilling a hole into the earth (or seabed) with a drilling rig that rotates a drill string with a drilling bit attached to the downward end. Usually the borehole is anywhere between about 5 inches (13 cm) to about 36 inches (91 cm) in diameter. As upper portions are cased or lined, progressively smaller drilling strings and bits must be used to pass through the uphole casings or liners, which steps the borehole down to progressively smaller diameters.
While drilling an oil or gas well, a drilling fluid is circulated downhole through a drillpipe to a drill bit at the downhole end, out through the drill bit into the wellbore, and then back uphole to the surface through the annular path between the tubular drillpipe and the borehole. The purpose of the drilling fluid is to lubricate the drill string, maintain hydrostatic pressure in the wellbore, and carry rock cuttings out from the wellbore.
The drilling fluid can be water-based or oil-based. Oil-based fluids tend to have better lubricating properties than water-based fluids, nevertheless, other factors can mitigate in favor of using a water-based drilling fluid.
In addition, the drilling fluid may be viscosified to help suspend and carry rock cuttings out from the wellbore. Rock cuttings can range in size from silt-sized particles to chunks measured in centimeters. Carrying capacity refers to the ability of a circulating drilling fluid to transport rock cuttings out of a wellbore. Other terms for carrying capacity include hole-cleaning capacity and cuttings lifting.
Both the dissolved solids and the undissolved solids can be chosen to help increase the density of the drilling fluid. An example of an undissolved weighting agent is barite (barium sulfate). The density of a drilling mud can be much higher than that of typical seawater or even higher than high-density brines due to the presence of suspended solids. The weight of pure water is about 8.3 ppg (990 g/l), whereas mud weights can range from about 6 ppg (720 g/l) to about 22 ppg (2600 g/l).
Sag of particulate weighting material, such as barite sag, has been a poorly understood phenomenon, especially in oil-based muds (“OBM”). Oil-based muds are typically used in moderate and high pressure and temperature environments. Sag may cause unwanted density variations in the circulating fluid, leading to well-stability or well-control issues. Sag is also of concern in highly deviated, directional and ERD (extended reach drilling) wells, and experiments have shown that the greatest influences of sag occur at well bore inclinations from 20° to 60° to the horizontal.
The large density variations created by sag can create wellbore management problems, and can even result in wellbore failure. Additionally, fluid sag can lead to sticking of drill pipe, difficulty in re-initiating or maintaining proper circulation of the fluid, possible loss of circulation and disproportionate removal from the well of lighter components of the fluid.
The issue becomes severe for highly deviated and complex wells. The ability to predict sagged fluid mud weight would be a crucial step in determining changes in torque, pump pressures, and bottom hole pressure excursions when flow is restarted due to a sag event.